US 20070183527 A1 Abstract Embodiments of the invention provide orthogonal Frequency Division Multiplexing-Space-time Block Codes (OFDMSTBC) mappings for code rate 1, 2 and 4 codes for 4 Transmit Antennas. To overcome the innate weakness of known codes, a new class of STBC codes is provided which is particularly suited for OFDM applications, although other applications are also contemplated. Codes for STBC mappings in the time direction, the frequency direction and a combined time frequency mapping for multiple antennas are provided. In OFDM applications, these codes exploit the properties of OFDM and FEC codes to substantially maintain the advantages of Alamouti codes. Furthermore, while the detailed examples given herein focus on four antenna applications, they can be easily extended to systems with more than four antennas.
Claims(40) 1. A method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by:
during n pairs of consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all antennas are used during the n pairs of consecutive transmission intervals and only one pair of antennas is active during a given transmission interval. 2. The method of mapping transmission symbols to the antennas using at least two different patterns for different transmission periods. 3. The method of 4. The method of 5. The method of mapping transmission symbols to the antennas using at least two different patterns for different transmission periods. 6. The method of 7. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*_{1} _{2}*_{3} _{4}*_{2} _{1}*_{4} _{3}*or
_{1} _{2}*_{3} _{4}*_{4} _{3}*_{2} _{1}*8. The method of 9. The method of 10. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*_{1} _{2}*_{3} _{4}*_{2} _{1}*_{4} _{3}*or
_{1} _{2}*_{3} _{4}*_{4} _{3}*_{2} _{1}*11. The method of 12. The method of 13. A method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols on each sub-carrier of a plurality of sub-carriers in an OFDM spectrum by:
for each transmission interval: on each of n pairs of adjacent OFDM sub-carriers, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all sub-carriers are used and only one pair of antennas is active during a given transmission interval for a given sub-carrier. 14. The method of 15. The method of 16. The method of 17. The method of 18. The method of mapping transmission symbols to the antennas using at least two different patterns for different active sub-carriers. 19. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*_{1} _{2}*_{3} _{4}*_{2} _{1}*_{4} _{3}*or
_{1} _{2}*_{3} _{4}*_{4} _{3}*_{2} _{1}*20. The method of 21-54. (canceled) 55. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*_{1} _{2}*_{3} _{4}*_{2} _{1}*_{4} _{3}*or
_{1} _{2}*_{3} _{4}*_{4} _{3}*_{2} _{1}*56. A method for mapping a space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by:
during two consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas. 57. The method of 58. The method of mapping transmission symbols to the antennas using at least two different patterns for different transmission periods. 59. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*_{1} _{3}*_{2} _{4}*_{3} _{1}*_{4} _{2}*or
_{1} _{4}*_{2} _{3}*_{3} _{2}*_{4} _{1}*60. A method for mapping a space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by:
for each transmission interval; on a pair of adjacent OFDM sub-carriers, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas. 61. The method of 62. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*_{1} _{3}*_{2} _{4}*_{3} _{1}*_{4} _{2}*or
_{1} _{4}*_{2} _{3}*_{3} _{2}*_{4} _{1}*63. A method for mapping a space-dine block code for a 2 n+1 antenna transmitter where n>=1, the method comprising transmitting transmission symbols by:
during k pairs of consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all antennas are used during at least one of the k pairs of consecutive transmission intervals and at least one transmit antenna is used during two of the k pairs of consecutive transmission intervals. 64. The method of 65. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*66. A method for mapping a space-time block code for a 2 n+1 antenna transmitter where n>=1 adapted to transmit transmission symbols by:
for each transmission interval: on k pairs of sub-carriers, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all antennas are used during at least one or the k pairs of consecutive transmission intervals and at least one transmit antenna is used during two of the k pairs of sub-carriers. 67. The method of _{1} _{2}*_{2} _{1}*_{3} _{4}*_{4} _{3}*68. A method for mapping a rate ¾ space-time block code for a 4 antenna transmitter, the method comprising transmitting 3 transmission symbols by:
during a pair of consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a first respective pair of antennas on a given sub-carrier and transmitting a respective Alamouti code block containing one transmission symbol on a second respective pair of antennas on a same sub-carrier such that all antennas are used during the pair of consecutive transmission intervals. 69. The method of _{1} _{2}*_{3}* _{2} _{1}*_{3}*_{3} _{1}*_{2}* _{3} _{2} _{1} 70. A method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by:
during a pair or consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas on a given sub-carrier such that all antennas are used during the pair of consecutive transmission intervals and each pair of antennas is active on a different sub-carrier. 71. The method of _{1} _{2}* _{2} _{1}*_{3} _{4}* _{4} _{3}*_{1} _{2}* _{3} _{4}* _{4} _{3}*_{2} _{1}*or
_{1} _{2}*_{3} _{4}*_{2} _{1}*_{4} _{3}*72. A method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 4 n transmission symbols by:
during a pair of consecutive transmission intervals, transmitting a respective Alamouti code block containing n transmission symbols on a respective pair of antennas on a given sub-carrier such that all antennas are used during the pair of consecutive transmission intervals. 73. The method of _{1} _{2}*_{5} _{7}*_{2} _{1}*_{6} _{8}*_{3} _{4}*_{7} _{5}*_{4} _{3}*_{8} _{6}*or
_{1} _{2}*_{5} _{7}*_{2} _{1}*_{7} _{5}*_{3} _{4}*_{6} _{8}*_{4} _{3}*_{8} _{6}*Description This application claims the benefit of U.S. Provisional Patent Application No. 60/558,566 filed on Apr. 2, 2004, U.S. Provisional Patent No. 60/563,802 filed on Apr. 21, 2004, and U.S. Provisional Patent Application Application No. 60/566,009 filed on Apr. 28, 2004. The invention relates to space-time transmit diversity systems and methods for OFDM (orthogonal frequency division multiplexing) applications. For open-loop fading channels, diversity is often considered the only choice to mitigate the fading effects and improve the system reliability. While FEC (forward error correction) coding provides valuable time diversity, for a slow fading environment, space diversity/polarization proves to be more effective. The objective of the so-called space-time codes is to maximize the diversity gain. There are basically two kinds of space-time codes: trellis codes and block codes. While trellis codes provide additional coding gain, they have drawbacks, namely: the decoder is more complex (since these codes work on the QAM symbols), and it is difficult to concatenate trellis space-time codes with powerful soft-decoding FEC codes, such as Turbo codes and convolutional codes. Space-time block codes (STBC) do not provide coding gain, but they are simple to decode (especially for those complex orthogonal design codes), and they can be naturally combined with other soft-decoding FEC codes. It is for these reasons that STBC (space-time block codes) are more practical and are widely adopted by numerous standards. The best known STBC code was invented by Alamouti and Tarokh (U.S. Pat. No. 6,185,258) where the code matrix is presented as in Table 1 below.
This code has several properties, which makes it perfect for space diversity. The signals are orthogonal; hence full diversity is achieved at the receiver side. The transmit power is balanced between the two antennas (and the two time slots); hence a low cost power amplifier can be used, which in turn reduces the modem cost. Its code rate is 1; hence no throughput is sacrificed. Its maximum likelihood decoder is very simple, which makes the cost of an optimal decoder negligible. Unfortunately, it was proven later that there are no such orthogonal codes existing for a system with more than two transmitter antennas. Therefore, the focus was shifted to the following two areas: designing orthogonal STBC codes with a code rate smaller than 1; and designing quasi-orthogonal STBC codes to maintain the property of code rate 1. While the first approach sacrifices the system throughput, the second approach sacrifices signal quality due to a loss of orthogonality. There are several variations of the codes falling into the above categories, with focus on other areas such as power balancing and code rate manipulation; but no codes with the beauty of the Alamouti have been found. Table 2 below is a first example of an STBC code for 4 transmit antennas (code-A). This is an orthogonal code having code rate ¾. This code strives to maintain the orthogonality of the codes, thereby the diversity order and signal quality.
With this code, each received signal has a diversity order of 4. However, one weakness of this code is its code rate loss, which must be compensated in FEC codes. In other words, due to the rate loss in STBC, the code rate in FEC must be higher. This can cause problem for those powerful trellis codes at higher code rate, whose coding gain loss becomes significant when punctuation becomes excessive. Note that this code does not achieve full power balance across transmit antennas. There are other codes available that are able to overcome this weakness. Table 3 contains a second example of an STBC code for 4 antenna applications (Code-B). This is a code rate 1 non-orthogonal code. This code strives to maintain the throughput of the system; thereby no coding gain will be lost at the FEC stage.
Since code-B is no longer an orthogonal code, it determinant suffers from mutual interference. Defining the equivalent channel matrix Q as
According to an aspect of the present invention, there is provided a method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by; during n pairs of consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all antennas are used during the n pairs of consecutive transmission intervals and only one pair of antennas is active during a given transmission interval. According to another aspect of the present invention, there is provided a method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols on each sub-carrier of a plurality of sub-carriers in an OFDM spectrum by; for each transmission interval: on each of n pairs of adjacent OFDM sub-carriers, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all sub-carriers are used and only one pair of antennas is active during a given transmission interval for a given sub-carrier. According to another aspect of the present invention, there is provided a method for mapping a space-time blocks code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by: during two consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas. According to another aspect of the present invention, there in provided a method for mapping a space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by: for each transmission interval; on a pair of adjacent OFDM sub-carriers, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas. According to another aspect of the present invention, there is provided a method for mapping a space-time block code for a 2 n+1 antenna transmitter where n>=1, the method comprising transmitting transmission symbols by: during k pairs of consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all antennas are used during at least one of the k pairs of consecutive transmission intervals and at least one transmit antenna is used during two of the k pairs of consecutive transmission intervals. According to another aspect of the present invention, there is provided a method for mapping a space-time block code for a 2 n+1 antenna transmitter where n>=1 adapted to transmit transmission symbols by; for each transmission interval: on k pairs of sub-carriers, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all antennas are used during at least one of the k pairs of consecutive transmission intervals and at least one transmit antenna is used during two of the k pairs of sub-carriers. According to another aspect of the present invention, there is provided a method for mapping a space-time block code for a 4 antenna transmitter, the method comprising selecting an appropriate code rate code from a code set of available code rate codes, wherein the code set is:
or
According to another aspect of the present invention, there is provided a method for mapping a space-time block code for a 4 antenna transmitter, the method comprising selecting an appropriate code rate code from a code set of available code rate codes, wherein the code set is:
or
According to another aspect of the present invention, there is provided a method for mapping a rate ¾ space-time block code for a 4 antenna transmitter, the method comprising transmitting 3 transmission symbols by: during a pair of consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a first respective pair of antennas on a given sub-carrier and transmitting a respective Alamouti code block containing one transmission symbol on a second respective pair of antennas on a same sub-carrier such that all antennas are used during the pair of consecutive transmission intervals. According to another aspect of the present invention, there is provided a method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 2 n transmission symbols by: during a pair of consecutive transmission intervals, transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas on a given sub-carrier such that all antennas are used during the pair of consecutive transmission intervals and each pair of antennas is active on a different sub-carrier. According to another aspect of the present invention, there is provided a method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2, the method comprising transmitting 4 n transmission symbols by: during a pair of consecutive transmission intervals, transmitting a respective Alamouti code block containing n transmission symbols on a respective pair of antennas on a given sub-carrier such that all antennas are used during the pair of consecutive transmission intervals. According to another aspect of the present invention, there is provided a method of transmitting a MIMO OFDM signal expressed in the form Y=HF(S), where Y Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. Preferred embodiments of the invention will now be described with reference to the attached drawings in which: In accordance with embodiments of the invention, methods are provided for mapping users to a multi-antenna transmit resource that enables the network to exploit the time-frequency diversity and multi-user diversity. In addition, the mapping also allows flexible radio resource management/allocation and provides different QoS based services. Within the same framework, SISO, MISO, MIMO capable users can be supported. By way of overview, each user can be mapped onto a different OFDM resource which might be a sub-channel, e.g. the AMC sub-channel and/or a diversity sub-channel. For MIMO users, preferably, multiple different space-time coding schemes are supported such as SM (spatial multiplexing) and STTD (space-time transmit diversity). On a continuous basis, there is a stream of OFDM symbols associated with each transmit antenna. Each user may be first mapped onto one or multiple OFDM symbols and each OFDM symbol may then be mapped onto its associated antenna. Such mapping also allows per-antenna rate control (PARC) to be performed in some embodiments. Each OFDM symbol may be mapped onto its associated antenna in the sub-carrier domain. For certain sub-carriers, if no specific user data is mapped, then a null assignment to such sub-carrier maybe fed into the corresponding antenna. A very simple example of what might be transmitted at a specific instant in time as a result of a particular OFDM symbol and antenna mapping is shown in For the particular time instant, the OFDM bandwidth is allocated in four distinct frequency bands F Each packet is to be mapped onto the four antennas using a selected mapping scheme. Various space-time mapping schemes are described in detail below. In some embodiments, multiple different schemes are available or a given number of transmit antennas and receive antennas. For example, for a 2×2 system, preferably STTD or SM (BLAST—Bell Labs Layer Space Time) can be selected. In other embodiments only a single scheme is implemented for each antenna permutation. Single antenna users use a single input single output (SISO) (which may involve PARC—Per-antenna rate control) transmission scheme. The first packet The second packet The third packet The fourth packet The fifth packet Finally, packet Generally, each packet can be mapped individually to some or all of the antennas. This enables MIMO and non-MIMO users to be mixed. In the above example, packets Please note that the flexible mapping of MIMO and non-MIMO users applied both in the context of “partial utilization” and “full utilization”. With partial utilization, a given base station only has access to part of the overall OFDM band. In this case, the sub-bands F For SISO users, a single band on a single antenna will be used. For a MIMO user the configuration is denoted as N The flexible structure illustrated by way of example in The particular example shown in Details of a method of performing transmitting from a plurality of antennas are defined in applicants' co-pending application no, <attorney docket 71493-1330 entitled ______> hereby incorporated by reference in its entirety. The method generally involves: dividing an available OFDM bandwidth of each antenna into sub-channels; defining a plurality of regions, each region being defined by a respective set of sub-channels and a defined number of OFDM symbols; defining a respective antenna mapping for each region and selecting one or more of the plurality of antennas to use for the region, the antenna mappings including at least one MIMO mapping; mapping content for at least one user to each of the regions using the respective antenna mapping; transmitting each region on the one or more antennas selected for the region. Another way to think of this is that each time-frequency block that is defined can have its own matrix. Once the matrix is specified, the number of antennas at the output is defined. For example, a 2×2 matrix requires two antennas; a 4×4 matrix requires four antennas. The matrix also determines, not necessarily uniquely, the number of different users that can be mapped. Particular examples are given in the tables below. The content for multiple users of course needs to be mapped in a manner that is consistent and non-conflicting. Also, each user needs to be informed of where/when its content will be transmitted. For each individual user, the antenna mapping enables STTD, SM and PARC transmissions for either the AMC sub-channel or the diversity sub-channel. In one embodiment, any one of six different mapping configurations can be applied to each individual user there including three 4-transmit antenna mappings, 2-transmit antenna mappings and a single antenna mapping. The uplink may include, for example, two modes: (1) STTD for dual transmit antenna capable SS and (2) Virtual-MIMO for single transmit antenna capable SS. Referring now to Specific Antenna Mapping Example A more specific example of a quasi-orthogonal Space-Time Coding Flexible mode Assignment for SISO, STTD and SM based on the above will now be detailed. Downlink open loop transmission with multiple antennas at a base station (BS) can be configured by several transmission modes. Assuming that N
For MIMO down link transmission, space-time coding is preferably employed. In one embodiment, a 4×4 quasi-orthogonal space-time transmit diversity (QOSTTD) code is used as a mother code for space-time coding, and can be punctured in time to optimize for different receive antenna configurations. The MIMO transmission and reception can be expressed by Y=HF(S), where Y A code suitable for a 4×1 configuration is: (STC Code Rate=1)
A code suitable 4×2 configuration (STC Code Rate=2, STTD), in-time puncturing the columns 3&4, 7&8 and 11&12 of F A code suitable a 4×4 configuration (STC Code Rate=4, spatial multiplexing), columns 1, 3 and 5 of F In a four transmit antenna system, an SS is preferably configured to receive the transmission of space-time coding of 4×1, 4×2 or 4×4 configurations (F For two transmit antennas, two transmission modes are preferably supported: space-time transmit diversity and spatial multiplexing. For a 2×1 configuration the following is an example of a code structure; (STC Code Rate=1)
For 2×2, 2×4 configuration: (STC Code Rate=2), puncturing the even columns of F In a preferred embodiment, the SS is configured to receive transmission of space-time coding of 2×1, 2×2 or 2×4 configurations, (F Details of the above codes and of example implementations of the fast feedback channel can be found in Applicant's co-pending application <Attorney Docket 71493-1329> filed ______, entitled “Methods for Supporting MIMO Transmission in OFDM Applications” hereby incorporated by reference in its entirety. A first embodiment of the invention provides a code sate 1 OFDM-STBC mapping for 4 Transmit Antennas. To overcome the innate weakness of the above two kinds of codes, a new class of STBC codes is provided which is particularly suited for OFDM applications, although other applications are also contemplated. In OFDM applications, these codes exploit the properties of OFDM and FEC codes to substantially maintain the advantages of Alamouti codes. Furthermore, while the detailed examples given here focus on four antenna applications, they can be easily extended to systems with more than four antennas. One property of FEC codes is their diversity effect within the code block. With this knowledge, the diversity order on each Quadrature Amplitude modulation (QAM) symbol can be relaxed to within one code block. Code rate 1 orthogonal STBC code designs need to achieve full diversity per QAM symbol, something which is difficult, if not impossible for a larger number of antennas than two. However, this is not necessary when the system is concatenated with soft-decoding based FEC codes. Therefore, new STBC codes are provided so as to provide a diversity order of two per QAM symbol, while still achieving full diversity with the help of FEC codes. Diversity order two means that each QAM symbol travels through two independently (multipath) fading channels. An example of a new STBC code is presented in Table 4 below (Code-C).
In Table 5A, another arrangement of an alternative antenna hopping pattern different than Table 4 is shown. Antenna hopping refers to a reordering of the mapping such that the same coding is performed, but that symbols are transmitted by different antennas. For example, in antenna hopping pattern 1 shown in Table 4 above, Antenna
In a preferred embodiment, multiple different patterns are employed over time, such as the above-introduced “hopping pattern-1” and “hopping pattern-2”. The effect of this is that for each group of symbols s Preferably, for the above embodiment, the mappings are used on a per OFDM sub-carrier basis. With Alamouti codes hopping across the transmit antennas, each QAM symbol has a diversity order of 2; with FEC coding being applied across all the STBC blocks, full diversity order is achieved. The STBC code rate is one, and each code STBC block is orthogonal. In order to achieve power imbalance for all the transmit antenna in time-domain, from Table 4, it can be noticed that at each instant only two transmitters are transmitting, which means for a constant power of P, each power amplifier (PA) can be designed to have 3 dB more dynamic range than when power balanced codes are used. Here, properties of OFDM can be employed. Preferably, different patterns are used for different sub-carriers such that an overall power balancing is achieved. This is detailed further below. For the specific example above, a first set of sub-carriers could be used to transmit pattern-1, and a second set of sub-carriers used to transmit pattern-2 at the same time. An OFDM signal has multiple sub-carriers, with each sub-carrier being treated as a flat fading channel. After IFFT processing occurs, of the type described in the examples of FIGS. In In some embodiments, the mapping code on a first set of sub-carriers have the mapping described above and a second set of sub-carriers will have a mapping that is an inverse of the described mapping, that is the location of the Alamouti blocks switch location in the time interval/antenna mapping shown in Tables 4, 5A and 5B. For example, on a first sub-carrier, during the first two transmit intervals, t and t+T, Antennas In the illustrated example, during a given pair of transmit intervals, all of the odd sub-carriers k, k+2, . . . , k+(n−3), k+(n−1) of one pair of antennas are active, and all of the even sub-carriers k+1, k+3, . . . , k+(n−2), k+n of the other pair of antennas are active. This then reverses for the following pair of transmission intervals. This is only one way to map the single sub-carrier solution onto a set of sub-carriers. In some embodiments different antennas are used for different sub-carriers (in frequency) to achieve power balancing in the time domain. This is OFDM specific, and is one of the reasons that the new code designs are attractive for practical implementation. A performance advantage in terms of SNR gain of OFDM-STBC Code-C is provided in some embodiments is shown in FIG. In addition to performance improvements achieved by the new codes, they also possess other additional merits. In some embodiments, the solution is particularly adaptable to OFDM arrangements, but may be suitable for other systems if the resulting power imbalance is acceptable. In some embodiments, such a construction is optimum and can be generalized to 2 n transmit antennas (no other R=1 full diversity code is known to date). This code also utilizes soft-decoding FEC codes to realize full diversity for the whole FEC code block. It is from STBC code-rate 1, full orthogonality of each STBC code block, and full diversity of the whole FEC code block point of view, that optimality is achieved. It was proven that the optimum code with code-rate 1, full orthogonality of each STBC code block, and full diversity for each transmitted symbol does not exist. In some embodiments, since in OFDM applications there are now three dimensions to map the Alamouti structure (essentially a 2 dimensional mapping), the time span for Alamouti mapping can be reduced by alternating the antenna or frequency dimension. It is noted that one basic assumption of STBC is that the channel does not change from one transmission to the next transmission. For the prior art codes describe above, each QAM symbol is transmitted four times, and this can cause performance loss in the case of mobility, where the basic assumption becomes less and less true. Obviously, the longer the time (or frequency) that elapses between the first and the last transmissions the greater the loss that will be resulted in. The new codes are well suited for this case, for each QAM symbol is only transmitted twice, consecutively. The above described code mappings are examples resulting from of a more general method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2. 2 n transmission symbols are transmitted during n pairs of consecutive transmission intervals, by transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all antennas are used during the n pairs of consecutive transmission intervals and only one pair of antennas is active during a given transmission interval. In some embodiments, this method is performed on a per-OFDM sub-carrier basis. Furthermore, the different antenna hopping patterns generally serve to map transmission symbols to the antennas using different patterns for different transmission periods. Code Rate 2 OFDM-STBC for 4 Transmit Antennas The fully orthogonal structure for a four antenna transmit case does not exist when the space-time coding rate is 2. However, a sub-optimal code structure can be constructed, as shown in Table 6. Table 7A and Table 7B provide alternative antenna hopping variants. Furthermore, while the detailed examples given here focus on four antenna applications, they can be easily extended to systems with more than four antennas.
The above described code mappings are examples resulting from of a more general method for mapping a space-time block code for a 2 n antenna transmitter where n>=2, 2 n transmission symbols are transmitted during two consecutive transmission intervals, by transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas. The three code rate 2 “hopping pattern” codes can be combined in various different ways to form a code set. Extension to the Frequency Direction Mapping The above STBC code construction is provided in the form that maps the Alamouti structure in the time direction, however, such an arrangement can be also mapped onto the frequency direction. Similar to the mapping in the time direction, an example of a power balance arrangement for the mapping for the frequency direction is shown in Examples of STBC codes for mapping the Alamouti structure in the frequency direction are listed in Table 8, Table 9A and Table 9B for code rate 1 and Table 10 and Table 11 are for code rate 2. Furthermore, while the detailed examples given here focus on four antenna applications, they can be easily extended to systems with more than four antennas.
The above described code mappings are examples resulting from of a more general method for mapping a rate one space-time block code for a 2 n antenna transmitter where n>=2. 2 n transmission symbols are transmitted on each sub-carrier of a plurality of sub-carriers in an OFDM spectrum, for each transmission interval, on each of n pairs of adjacent OFDM sub-carriers, by transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas such that all sub-carriers are used and only one pair of antennas is active during a given transmission interval for a given sub-carrier. In some embodiments, during a given transmit interval, sub-carriers are active in pairs, alternating between pairs of transmit antennas. Furthermore, the different antenna hopping patterns generally serve to map transmission symbols to the antennas using different patterns for different sub-carriers.
The above described code mappings are examples resulting from of a more general method for mapping a space-time block code for a 2 n antenna transmitter where n>=2. 2 n transmission symbols are transmitted for each transmission interval on a pair of adjacent OFDM sub-carriers, by transmitting a respective Alamouti code block containing two transmission symbols on a respective pair of antennas. Extension to 3 Transmit Antennas The new codes can be extended to odd numbers of transmit antennae as well. Table 12 and Table 13 are embodiments of the transmit antenna OFDM-STBC code in the time direction and in frequency direction, respectively for an example with three antennas. In these embodiments the code rate is reduced to ⅔, however, however full orthogonality is achieved.
Code Rate 4 OFDM-STBC for 4 Transmit Antennas Code rate 4 arrangements become the typical parallel spatial multiplexing known as BLAST. In Table 14 and Table 15 embodiments of a mapping of 4 transmit antenna BLAST in time and in frequency are listed. Furthermore, while the detailed examples given here focus on four antenna applications, they can be easily extended to systems with more than four antennas.
OFDM-STBC Code sets with Antenna Hopping Various embodiments of 4 antenna transmit OFDM-STBC code sets, are listed in Tables 16A, 16B, and 16C.
Tables 16A, 16B, and 16C are embodiments of code sets for space-time mappings in the time direction. Similarly, the space-time mappings could be expressed in the frequency direction as well, as shown in Tables 16D, 16E, and 16F.
A specific Alamouti coded has been used in the examples. It is to be understood that any code having the Alamouti structure can be employed. Decoder Complexity A decoder complexity for the 4 transmit STBC of Code-C compared with the prior art codes is listed in Table 17. It can be seen that the complexity of the OFDM-STBC Code-C is 16 times less than the prior art codes.
Space-Time Frequency Mapping for Multiple Transmit Antennas As described above, the space-time coding can be mapped onto either the time direction or the frequency direction. In other embodiments, a mapping is provided that utilizes both time and frequency mapping, while providing for the entire antenna transmission on each sub-carrier based on the code format. Advantages of this combined time-frequency mapping include minimizing the time-frequency span to ensure the space-time code span is within both a coherent-time and coherent-frequency range. The following examples are provided in particular for 4 transmit antennas, but it is to be understood that the space-time frequency mapping approach can be generalized fox systems with a different number of transmit antennas.
Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. Patent Citations
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